(1) Field of the Invention
The invention lies in the technical field of electrical installations for rotary wing aircraft. More specifically, the invention relates to storing energy for such electrical installations.
(2) Description of Related Art
A rotary wing aircraft is conventionally provided with at least one main rotor for providing it with lift and possibly also propulsion, and generally with an antitorque tail rotor specifically for opposing the yaw torque exerted by the main rotor of the fuselage of the aircraft and also for controlling yaw movements of the aircraft.
In order to drive the main rotor and the tail rotor in rotation, the aircraft includes a power plant that may include one or more fuel-burning engines.
It should be observed that throughout this text the term “fuel-burning engine” or just “engine” for short is used to cover both turboshaft engines and any piston engine suitable for use in such a power plant. The term “engine” is to be contrasted with the term “electric motor” or just “motor” for short relating to motors driven by electrical power.
Furthermore, in the general field of storing electrical energy, thermopiles are known.
Thus, Document EP 1 059 134 describes thermopiles that are thus used mainly in the aviation and space industries or in emergency safety systems where a reliable backup energy source is required, e.g. in the nuclear, oil, or building industries. Thermopiles are not rechargeable, and prior to being triggered they are inert, thus enabling them to be stored without maintenance, sometimes for as long as 20 years, while remaining usable at any instant with a response time that can sometimes be less than a few tenths of a second. The use of thermopiles is increasing in all fields where there is a need for energy to be available immediately and reliably, even after a storage time that might be very long. Such thermopiles incorporate metal powder, e.g. prepared iron powder, which presents a spongy and filamentary structure. The powder is used in heating compositions for thermopiles.
Various documents propose incorporating one or more energy storage thermopiles in a rotary wing aircraft.
Document FR 2 994 687 describes providing a pilot of a rotary wing aircraft with assistance during a stage of flight in autorotation. The aircraft has a hybrid power plant with a fuel-burning engine, at least one electrical machine, and a main gearbox. By way of example, onboard electrical energy storage may comprise a supercapacitor type capacitor capable of delivering high power for a limited time, a thermopile that requires heat to be delivered in order to supply power, or indeed a rechargeable battery. In flight, the main rotor is driven at a nominal speed of rotation by the hybrid power plant, i.e. a power plant made up of at least one fuel-burning engine and at least one electric motor, such that during an in-flight monitoring step a monitored parameter is measured in order to detect a failure of the engine, if any. When a failure is detected, the electric motor is operated to deliver auxiliary power to the main rotor, thus enabling the pilot to be assisted during flight in autorotation as a result of the failure, thus providing the aircraft with an additional margin for maneuver.
Document FR 2 997 382 describes in-flight control of the operation of fuel-burning engines of a rotary wing aircraft, by means of an electronic engine control unit (EECU) in order to detect an engine failure, if any. An engine is considered to have failed when at least one other engine is being used to deliver power at a contingency rating. The monitoring determines a monitored value of a parameter of the aircraft and a detection threshold for detecting total loss of power. Thereafter, comparing the monitored value with the detection threshold identifies a risk of total loss of power. This loss appears as soon as at least one engine is called on to supply power above a predetermined power level. If a failure is detected with the threshold being crossed, action is taken to ensure that sufficient auxiliary power is supplied to enable the aircraft to be operated safely, e.g. with each engine not delivering power above the predetermined power. For example, the storage means may comprise at least one rechargeable battery, a thermopile, or indeed a supercapacitor.
It can also be advantageous to use one or more thermopiles for the power plants of rotary wing aircraft, such as that described in Document FR 2 952 907. The power plant has a single fuel-burning engine, a main gearbox (MGB) suitable for driving the rotary wing, and a tail gearbox for driving an antitorque rotor. The installation includes a first electric motor mechanically connected to the main gearbox and a second electric motor mechanically connected to the tail gearbox.
Document WO 2012/059671 describes a helicopter having two turboshaft engines and a regulator system. Each of the two turboshaft engines has a gas generator and a free turbine with means suitable for activating the gas generator starting from a super-idle speed. Rotary drive means, gas generator accelerator means, and ignitor means of almost instantaneous effect are provided in the architecture. Those means are in addition to an emergency mechanical assistance device that makes use of an independent onboard energy source. Almost instantaneous ignition is provided as a function of the conditions and stages of flight of the helicopter depending on its mission profile, e.g. during transient conditions or in the event of a failure of the engine in use by reactivating the other engine. For example, when an oversized turboshaft engine that is being used on its own during stages of cruising flight suffers a failure, another engine, which is small, is rapidly reactivated via its emergency assistance device. The electrical equipment connected to the gas generator of this engine starts it and accelerates until its speed of rotation is in an ignition window for the combustion chamber, and then once the combustion chamber has ignited, the gas generator is again accelerated, but in conventional manner. Under super-idle conditions with the combustion chamber extinguished, it is possible to trigger additional ignition of the combustion chamber, i.e. ignition additional to conventional ignition.
Furthermore, Document XP055279373 “ASB—domains de performance des piles Thermiques” [ASB—performance domains of thermopiles] describes a diagram suitable for visualizing the current state of a performance domain of thermopiles. That document describes ranges of values for thermopiles with power going up to 9 kilowatts (kW), with bursts to 25 kW, and specific energy up to 120 watt hours per kilogram (Wh/kg) associated with high discharge rates, a operating duration lying in the range 0.5 seconds (s) to 2 hours (h), and an activation duration starting from 30 milliseconds.
From the above, and with a reasonable thermopile specification, it can be seen that, for improving power plants, it would be useful to make sufficient usable power available for conditions and stages of flight that might occur during certain missions.
In addition, it would be advantageous to use one or more thermopiles for pieces of equipment on board rotary wing aircraft other than power plants, however storing electrical energy is one of the main brakes on electrifying such aircraft.
More generally, the use of electrical energy for rotary wing aircraft provides several advantages, in particular in terms of providing a reserve of energy during certain critical stages of flight such as an engine failure or emergency situations during which maintaining emergency functions increases the safety of the aircraft.
Furthermore, the increasingly strict standards concerning flight safety, pollution emission, and reducing sound nuisance are favorable for this type of energy. Likewise, for onboard equipment such as flight controls, it is more and more frequent to have recourse to electrical devices, for reasons of simplifying design and maintenance, and for reasons of weight and size, in particular.
Consequently, the electrification of rotary wing aircraft incorporating thermopiles is promising.
Nevertheless, batteries are heavy and indeed very heavy if a large quantity of electrical energy needs to be stored, and supercapacitors can supply a large amount of electrical power only over a very limited length of time.
Although thermopiles are for single use only and have a limited operating duration after activation, incorporating them appears to be favorable in certain applications in the field of rotary wing aircraft.
Nevertheless, in practice, several technical problems arise when one or more thermopiles are incorporated in energy storage onboard a rotary wing aircraft.
Thus, incorporating thermopiles in energy storage on board a rotary wing aircraft involves providing thermal protection suitable for keeping such thermopiles in a temperature range that guarantees an optimum supply of energy and in particular of usable energy, while ensuring that structures adjacent to the energy storage of the aircraft do not run the risk of being exposed to excessive heating. For example, composite materials are increasingly present in such aircraft, but they present mechanical properties that are good only below certain temperatures.
For example, thermopiles generally include trigger devices, generally pyrotechnic devices, that are fired electrically when activating them. It is therefore appropriate to control the temperature rises that are due to such devices.
Thus, controlling the temperature of thermopiles must enable thermopiles to be maintained in a relatively limited range of temperatures during each design supply duration, so that appropriate quantities of energy can be made available at the opportune moment.
Furthermore, prior to activation, thermopiles present electrical resistance that is very high, being measured in megohms. However, after activation, thermopiles present tiny resistance that is measured in tenths of an ohm.
In an electric circuit including an energy storage system, it is common practice to use energy converters including filter stages of the capacitive type. When the storage device is put into operation, there is then a large inrush of current for charging the capacitors, and that can damage certain elements in the electrical circuit system, in particular power contactors. It is then appropriate to use a dedicated pre-load circuit comprising a contactor in series with a resistor so as to limit excessive inrush currents. Such additional circuits increase the weight of the onboard electrical equipment, make it more complex, and present a cost that is not negligible. The use of a thermopile makes it possible to avoid the power contactor and the pre-load circuit since current is limited automatically on activation by the internal resistance of the thermopile, which decreases continuously and sufficiently slowly.
A technical problem posed by incorporating thermopiles in rotary wing aircraft is to be able to determine the specifications involving such thermopiles in a manner that is simple, accurate, and without extra cost or extra weight that would be harmful to the aircraft as whole.
Thus, when incorporating thermopiles in a rotary wing aircraft, prior calculation of values for the time it takes to use thermopiles, the duration during which they provide energy, and the power values of such thermopiles is complex but important for determining the advantage of such integration.